Instantaneous Increase in Mean Circulatory Pressure and Cardiac Output at Onset of Muscular Activity

In dogs with their spinal cords cut, sudden maximal muscular contraction in the lower half of the body caused the mean circulatory pressure to rise instantaneously to three times normal and the cardiac output to rise to an average of 40 per cent above normal. This instantaneous effect was not blocked by sympathetic blockade with hexamethonium but was blocked by skeletal muscle blockade with decamethonium. Furthermore, because of the transected cord, no cardiac reflex effects were observed. Therefore, it was concluded that skeletal muscle activity compresses the intra-muscular and intra-abdominal vessels and thereby increases the mean circulatory pressure. Evidence is presented to show that this, in turn, translocates blood into the heart and that the heart then responds in conformity with Starling's principle of cardiac adaptation to increase the cardiac output.